Principles Of Gait Analysis

Gait analysis (also referred to as motion analysis or biomechanics) using cameras is not a new idea. It wasn't too long after the advent of photography that Eadweard Muybridge "freeze-framed" his first horse in the late 1880's and showed the gallop to be a four-beat gait. Since then, advances in technology have brought about the ability to film horses with high-speed, motion picture cameras or video cameras capable of recording hundreds of frames per second (compared to the rate of 30 frames per second for standard video cameras). Later, when the film or tape is played back very slowly, frame by frame, details of the action emerge that would escape even the best observer, since the human eye cannot distinguish events occurring in less than 1/10th of a second.

Translating these action details into numbers (quantifying the data) is the next step in motion analysis. A multitude of measurements can be taken, depending on your interest and purpose. For example, suppose you would like to know something about your horse's stride aharacteristics. By measuring the distance on the screen between successive imprints of the same hoof (or better yet, taking the average distance between successive imprints of all four hoofs) and comparing it to a marker of known length, you can calculate stride length. If the camera is equipped with a timer, figuring stride duration becomes a simple matter (otherwise, since the recording rate is known in terms of frames per second, you can derive stride duration by counting the number of frames it takes to complete one stride). From there, stride frequency and velocity can be computed.

Joint angles, vertical movement of the croup and withers, breakover time, the relative positions of the limbs at each phase of the stride and the timing of each hoof placement are other calculations that may be important in studying the biomechanics of your horse in motion. In fact, depending on the angle of the camera(s) and the points of reference, the number of ways in which a horse's gait can be quantified is almost unlimited. And if you take the horse over a force plate, you've got a whole new set of variables to measure. Not only can you examine the geometry of movement as the limbs travel through space (kinematics), but the force plate allows you to study the forces that cause movement (kinetics).

Although quantifying gait characteristics makes it possible to answer all kinds of how much, how fast, how far, how high, how long and how wide questions, gathering the data necessary to do so can be a very time-consuming and tedious process. Enter computer technology, and motion analysis picks up speed. Digitizing translates the location of anatomical landmarks into coordinates, and these numbers are stored by the computer and fed into equations that calculate the variable of choice. Specialized software coupled with a computer's renowned ability to perform complex number-crunching faster than a speeding math teacher allows the raw data to be transformed into graphs, charts and even moving stick figures.